Most of us think of Einstein's theory of special relativity in terms of vast distances and massive objects - black holes, space travel, planetary motion.
But for the first time, a team of researchers at Washington State and Brown universities has observed how relativity alters chemical bonds involving heavy atoms, where electrons move at nearly light speed.
Using photoelectron spectroscopy, scientists at Brown captured experimental evidence of the effect in a molecule made of carbon and bismuth, one of the heaviest elements on the periodic table. WSU chemist Kirk Peterson confirmed the finding with advanced computational modeling: relativity had fundamentally reshaped the molecule's chemical bonds in a way scientists had long predicted but never directly observed.
Instead of an ordinary triple bond, made up of one "sigma" and two "pi" bonds, scientists observed bonds that were neither sigma nor pi, but a blurred hybrid.
"This is something you can predict from theory, it's just never been observed before experimentally," said Peterson, a professor in the WSU Department of Chemistry. "Once you turn on relativistic effects that are really strong, it destroys some of the symmetry of the molecule in the sense that now those two pi bonds actually become two different types of bonds."
The research, published in the journal Science, could have implications for modern synthetic chemistry - the creation of complex molecular structures used to pursue innovations in pharmaceuticals, sustainability, energy storage and other fields.
The collaboration on the project grew from relationships formed decades ago at the WSU Tri-Cities campus, where Peterson and physicist Lai-Sheng Wang first got to know each other as faculty members with joint appointments at the Pacific Northwest National Laboratory in the 1990s. Peterson moved to the Pullman campus in 2002, and Wang went to Brown in 2009.
"Lai-Sheng was essentially next door to me or downstairs from me, and so we knew each other very well," Peterson said. "We've always said we should collaborate more, but it never really worked out until now. So this was really kind of coming back together again and finding something we could work on, which was great for both of us. It's fun - it's come full circle."
The theory of special relativity describes how time and space is altered when objects travel near the speed of light. It is often thought of in the context of space travel, but electrons in some heavy molecules travel at relativistic speeds due to the effects of their large nuclear charge.
"It's really unique to heavy elements," Peterson said. "This is when the effects of special relativity become very strong. When you have very heavy elements like bismuth, the nuclear charge is so big that the electrons close to the nucleus are now approaching the speed of light and so they're affected by relativity."
Students in Wang's lab at Brown got the first glimpse of these effects while pursuing a different research subject - it was a bit of scientific serendipity. They followed up by conducting spectroscopy experiments on molecules that had been cryogenically cooled to subdue internal energies within the molecule and get a clearer picture of what happened when an electron was knocked off.
"This is not something we aimed at from the start," Wang said. "We were actually interested in boron clusters in my lab, but this showed up in our experiment. I give a lot of credit to my students. They were really analyzing the data and thought deeply about the relativistic effects, and then, at some point, we said, 'OK, we need to talk to Kirk.'"
Chemical bonds literally knit the world together - binding hydrogen and oxygen atoms to form water molecules, to use a simple example. Bonds are formed by atoms sharing electrons, and some elements form double or triple bonds.
Bonds come in two forms: sigma, which are strong, head-to-head bonds, and pi, which are side-by-side and weaker. This holds true in most elements, but toward the bottom of the periodic table, where the heavy elements dwell, things become fuzzier. The electrons' spin and orbit are altered, and the bonds no longer follow the textbook forms.
"The boundary between a sigma bond and a pi bond is now sort of smeared," Wang said. "We still have three bonds, but we don't strictly have a sigma or a pi anymore."
The findings reflect fundamental truths about the scientific process: It's a long game, with new discoveries building on the foundation of past knowledge over decades, and the process is often driven by scientists finding things they weren't looking for.
"This is science, right?" Peterson said. "This is the happy accident that suddenly you discover something new you weren't looking for. And you look a little deeper, and you find something really interesting is going on."
